J.G. Chen

Transport critical current density and microstructure in extruded YBa2Cu3O7−x wires processed by zone melting

YBa2Cu3O7−x compounds were extruded into long wires with the diameter of 1 mm after sintering. The sintered wires were subsequently zone melted to develop a highly textured microstructure. Magnetization experiments at 77 K indicated a Jc value of 1×105 A/cm2 at 1 T. Transport measurements at 77 K showed a greatly enhanced field dependence of the critical current density. Transmission electron microscopy revealed an important grain‐boundary feature which eliminated the weak‐link behavior. Large amounts of dislocations have also been found in the zone‐melted sample which may contribute to flux pinning in the system.

Transport-critical-current behaviour and grain boundary microstructure in YBa2Cu3Ox

The microstructures of polycrystalline YBa2Cu3Ox samples were varied through different sintering mechanisms to study transport-current characteristics in magnetic fields. TEM experimental results indicate that the transport critical current in polycrystalline YBa2Cu3Ox is closely correlated with grain boundary microstructures. Specifically, the Jc(H) is severely suppressed as the grain boundaries are contaminated with the second phases. The field required for the Jc(H) values to decrease to zero is also strongly affected by the grain boundary microstructure. The critical-current behaviour and closely related microstructural changes are explained with Josephson weak-link and flux-flow models.

A superconducting coil made of filamentary composites

Abstract By a swaging technique we have constructed a high- T c superconducting coil of filamentary composites. The swaged wires are highly flexible after sintering and can carry a transport critical current density of 3500 A/cm 2 at 77 K and zero magnetic field. Fine and homogeneous filaments with an average diameter of 100 μm are well embedded in the silver matrix after thermal processing. The constructed coil consists of 34 turns which are insulated with varnish. Detailed processing and characterization data for the fabrication of the coil are reported.

A TEM study of grain boundaries in zone-melted YBa2Cu3Ox

Abstract Grain boundaries in zone-melted YBa 2 Cu 3 O x have been studied in detail by means of transmission electron microscopy. Tilt and twist types of low-angle grain boundaries have been observed, which are composed of both edge and screw dislocations. These boundaries possess high crystal coherence and intimate contact. The low-angle boundaries are also found to be free of second phases. Direct dc transport critical current density, J c , at 77 K shows great improvement in the zone-melted sample compared with that of a conventionally sintered sample. The relationship between the enhanced J c and the low-angle grain boundaries is dicussed.

Synthesis of 85 K BiSrCaCuO superconductor

Abstract Synthesis of the compound Bi 2 Sr 2-y CaCu 2 O x , 0≤y≤0.5, was attempted from 10 4 to 10 6 Pa oxygen pressure. Lower pressures and temperatures less than 800°C proved most effective in forming the 85-K superconducting material. A Sr-deficient stoichiometry exhibited highest phase purity. Results suggest, however, that presence of a small amount of a phase such as the low-temperature superconductor, Bi 2 Sr 2 CuO 6 , has little effect on the resulting superconducting properties.

Synthesis of TlCaBaCuO superconductors

Abstract Starting compositions of Tl 2 Ca 2 Ba 2 Cu 3 O x , TlCa 2 Ba 2 Cu 3 O x , and TlCa 3 BaCu 3 O x were used to synthesize Tlue5f8Caue5f8Baue5f8Cuue5f8O superconductors. The atmosphere was pure oxygen of pressure 10 5 to 6 × 10 5 Pa, and the temperature range was 890 to 940°C. Chemical analysis, X-ray diffraction, differential thermal analysis, scanning electron microscopy, and resistivity and magnetic susceptibility measurements revealed that the TlCa 3 BaCu 3 O x composition formed the most phase pure compound. The highest T c was 120 K.

Critical currents, magnetic relaxation and microstructure in zone-melted YBa/sub 2/Cu/sub 3/O/sub x/

Extruded YBa/sub 2/Cu/sub 3/O/sub x/ wires have been zone-melted to develop highly textured microstructures. The zone-melted wires exhibit large magnetic hysteresis and transport J/sub c/ values relative to conventional bulk YBa/sub 2/Cu/sub 3/O/sub x/. The transport J/sub c/ in the zone-melted wire, in magnetic fields up to 1.5 T, has been greatly improved at 77 K. Transmission-electron-microscope evidence suggests that this enhancement of J/sub c/ may be connected with small-angle grain boundaries. The magnetic hysteresis loop at 77 K in the zone-melted wire is largely increased relative to that for a sintered specimen. This increase in the hysteresis loop may possibly be attributed to a high density of dislocations acting as strong pinning centers.

Critical current density, flux creep, and microstructure in a Bi-Sr-Ca-Cu-O system

We measured magnetization hysteresis and zero-field-cooled magnetic relaxation for liquid-quenched Bi--Sr--Ca--Cu--O samples at various temperatures. We found that there is a close correlation between the magnetic behavior and the microstructures of the samples. The magnetization hysteresis greatly increases in the samples with a large amount of crystal defects such as precipitates and stacking faults in the matrix of the material. We also found that the flux creep is enhanced as a result of these microstructural changes. 6 refs., 3 figs.

High {Tc} superconductor tunnel junctions for photon detectors

SIN and SIS quasiparticle tunnel junctions on high TC superconductors (HTS) offer the possibility of low-noise, heterodyne, photon detection in the THz regime. We report progress on the development of such junctions using mechanical point contacts. In general, these contacts display the optimum characteristics that can be obtained from HTS native-surface tunnel barriers. The bismuthate, Ba1-xKxBiO3, (TC equals 25 K) displays ideal, BCS, quasiparticle characteristics at T equals 4.2 K however, at temperatures T approximately equals TC/2 there is evidence of strong quasiparticle damping which may inhibit device performance. The cuprates typically display non-ideal quasiparticle characteristics including large sub-gap conductances. Recent data for the new Hg-based cuprates (TC equals 96 K) are promising in that they exhibit very low and flat sub-gap conductances as expected from a BCS density of states. Proximity effect tunnel junctions on Bi2Sr2CaCu2O8/Au bilayers have been studied using an In tip. An induced energy gap has been consistently observed in the Au layer and the data can be understood using the McMillan model. A few junctions show much improved sub- gap characteristics compared to ones made directly on the BSCCO surface and indicate that this approach may be suitable for mixer development.

Hyper- and Hypobaric Processing of T1-Ca-Ba-Cu-O Superconductors

Tl-based superconductors of initial composition Tl:Ca:Ba:Cu equal to 2:2:2:3 and 1:3:1:3 were heated in oxygen at pressures of 10{sup 4} to 6 {times} 10{sup 5} Pa. The 2:2:2:3 composition formed primarily the 2-layer superconductor with zero resistance from 77 to 104 K. The 1:3:1:3 composition formed nearly phase pure 3-layer superconductor with a maximum zero resistance temperature of 120 K. Application of hyperbaric pressure influenced phase purities and transition temperatures slightly; phase purities decreased significantly with application of hypobaric pressures. 10 refs., 1 fig.